In modern vehicles such as automobiles, aircraft, and marine vessels multiple and different types of microphones are utilized for different applications. For example, in automobiles directional microphones are used in speech recognition applications such as hands-free cellular telephone communications or voice activated instrument control. For these high quality in-vehicle speech applications, the most common microphone is the directional (first order gradient) microphone. Directional microphones that have polar response shapes such as cardioid, if oriented with their maximum response axis oriented towards the talker, do a good job of providing speech pickup while rejecting noise arriving from sources located away from the talker. Further rejection of low-frequency noise is achieved by a microphone high-pass frequency response characteristic which rolls-off sharply below the speech frequency range. In noisy environments, such as automobiles, this rejection of environmental noise results in increased signal-to-noise ratio which yields improved communication sound quality and better speech recognition scores as compared to a signal provided by a similarly located omni-directional microphone.
Additionally, and in contrast to the above requirements for high-quality in-vehicle speech microphones are the requirements for microphones intended to provide signals corresponding to the ambient noise in a vehicle. These in-vehicle microphones are typically used to provide an input signal to a system intended to reduce vehicle interior noise and/or to compensate loudspeaker volume in accordance with fluctuations in vehicle interior noise. In the latter application, these microphones are used to help create an apparently uniform loudspeaker level which tracks ambient noise level fluctuations and eliminates the need for manual loudspeaker volume adjustments by the listener. To facilitate good ambient noise pickup, unlike speech microphones, microphones in this application should have an omni-directional characteristic as well as flat frequency response extending to low frequencies, below the speech range.
Due to the conflicting requirements with respect to microphone directionality and frequency response, one microphone cartridge cannot adequately be employed for both speech recognition and ambient noise detection. The current state of the art is to use two physically separate microphones, each optimized for its intended use. However, this practice is clearly an expensive alternative.
Thus, it would be an advancement in the art to provide a single apparatus that simultaneously supports both high quality speech applications such as hands-free cellular phone communication and ambient noise sensing. Furthermore, it is desired that the apparatus be cost effective, and contained in a housing that is similar in size to an existing single cartridge microphone enclosure.